1. Field of the Invention
The present invention relates to a method of controlling the band-width of a dual mode band-pass filter for use as a band filter in a communication device operated in a microwave band to a millimeter wave band, and also relates to a dual mode band-pass filter.
2. Description of the Invention
Various conventional band-pass filters, for use in a high frequency region, have been proposed, for example, in MINIATURE DUAL MODE MICROSTRIP FILTERS, J. A. Curtis and S. J. Fiedziuszko, 1991 IEEE MTT-S Digest.
FIGS. 13 and 14 are schematic plan views showing conventional dual-mode band-pass filters, respectively.
In a band-pass filter 200 shown in FIG. 13, a circular conductive film 201 is provided on a dielectric substrate (not shown). An input-output coupling circuit 202 and an input-output coupling circuit 203 are coupled to the conductive film 201 at an angle of 90xc2x0 with respect to each other. A top-open stub 204 is provided at a location so as to define a center angle of 45xc2x0 relative to the location where the input-output coupling circuit 203 is disposed. Thereby, two resonance modes having different resonance frequencies are coupled. As a result, the band-pass filter 200 operates as a dual-mode band-pass filter.
Moreover, in a dual-mode band-pass filter 210 shown in FIG. 14, a substantially square conductive film 211 is provided on a dielectric substrate. Input-output coupling circuits 212 and 213 are coupled to the conductive film 211 at an angle of 90xc2x0 with respect to each other. The corner portion positioned at an angle of 135xc2x0 relative to the input-output coupling circuit 213 is cut away. With the cut away portion 211a, the resonance frequencies of the two resonance modes are different. The two resonance modes are coupled to each other, and thereby, the band-pass filter 210 operates as a dual-mode band-pass filter.
Moreover, a dual-mode band-pass filter having a circular ring-shaped conductive film instead of the circular conductive film is disclosed in Japanese Unexamined Patent Application Publication No. 9-13961 and Japanese Unexamined Patent Application Publication No. 9-162610. That is, a dual mode filter is disclosed in which a circular ringshaped ring-transmission line is provided, and input-output coupling circuits are arranged to form a center angle of 90xc2x0 therebetween, in addition to those in the dual-mode band-pass filter shown in FIG. 13. Moreover, a top-open stub is provided in a portion of the ring-shaped transmission line.
In each of the conventional dual-mode band-pass filters shown in FIGS. 13 and 14, the two-stage band-pass filter is constructed to include one conductive film pattern. Accordingly, the band-pass filters are miniaturized.
However, in two-stage band-pass filter having circular or square conductive film patterns, the input-output coupling circuits separated from each other by the above-mentioned particular angles are coupled. Therefore, it is impossible to enhance the coupling degree, and a wide transmission band cannot be achieved.
Moreover, in the band-pass filter shown in FIG. 13, the conductive film 201 has a circular shape. In the band-pass filter of FIG. 14, the conductive film 211 has a substantially square shape. That is, the conductive films are limited to these particular shapes. Accordingly, the design flexibility is greatly reduced.
Moreover, each of the above-described band-pass filters has a frequency band that operates in only one resonance mode. Thus, it is difficult to control the frequency band, due to the restrictions of the circular or square conductive film shapes.
To overcome the above-described problems with the prior art, preferred embodiments of the present invention provide a method of controlling the band-width of a dual-mode band-pass filter, in which the above-described defects of the conventional techniques are eliminated, miniaturization is achieved, reduction in size and realization of a wide band-width is achieved, and the design flexibility is greatly increased. Also, preferred embodiments of the present invention provide the dual-mode band-pass filter produced by this method.
According to preferred embodiments of the present invention, a dual-mode band-pass filter includes a dielectric substrate, a frame-shaped electrode pattern provided on one main surface of the dielectric substrate or inside the dielectric substrate, the frame-shaped electrode pattern including a line-shaped electrode having a substantially constant line-width from the starting point to the end point, the starting point and the end point being connected to each other, a ground electrode provided inside the dielectric substrate or on a main surface of the dielectric substrate and opposed to the frame-shaped electrode pattern via a portion of the dielectric substrate, and input-output coupling circuit electrodes coupled to the frame-shaped electrode pattern, at least one of a capacitance loading portion and an inductance loading portion being provided in a portion of the line-shaped electrode such that two resonance modes having different resonance frequencies and being generated at the frame-shaped electrode pattern are coupled to each other.
Preferably, the frame-shaped electrode pattern has a substantially rectangular or rhombic electrode pattern having four sides.
Also, preferably, the electrode widths of two adjacent sides of the four sides are different from each other, and the electrode widths of two opposed sides of the four sides are the same. Convex portions that function as the capacitance adding portions or concavities which function as the inductance adding portions are provided on two opposed sides of the four sides. Furthermore, the electrode lengths of two adjacent sides of the four sides are different from each other, and the electrode lengths of two opposed sides thereof are the same. The electrode including at least one side of the four sides preferably has a tapered shape.
Also, preferably, at least one corner portion of the four corner portions of the substantially rectangular or rhombic electrode pattern is bent.